Diesel engines normally operate at air to fuel ratios above stoichiometric. Emissions of nitrogen oxides and particulates from diesel-fueled vehicles may be significant. Emissions from diesel vehicles are subject to regulation in both the United States and Europe.
Both the United States and Europe have enacted regulations with strict limits on nitrogen oxides (NOx) and particulate matter (PM) emissions from diesel engines. As of 2007, the PM limit for US trucks was 0.01 g/bhp-hr, and the NOx limit was about 2 g/bhp-hr. For the 2010 standard, the PM limits are the same, however the NOx limit has been reduced almost 10 times. In the 2010 standard, the NOx limit is 0.2 g/bhp-hr for NOx will take full effect. Starting in 2011, engines for non-road applications will have to change their emission limits to 0.02-0.03 g PM/bhp-hr and 0.4 gNOx/bhp-hr. All off-road engines with higher than 56 kW power output should meet these standards in 2014.
Nitrogen oxides are also present in exhaust gases from stationary sources such as power plants, industrial processes, gas turbines, etc.
One method of removing NOx in exhaust gas is to contact the exhaust stream with a reductant such as ammonia in the presence of a catalyst at elevated temperature. The catalyzed reaction of the reductant with NOx is referred to as selective catalytic reduction (SCR). Urea, ammonium hydroxide, ammonium formate, and other nitrogen-containing chemicals can also be used as a source of ammonia.
Traditional ammonia SCR catalysts are based on vanadia/titania. Imanari, et al. (U.S. Pat. No. 4,833,113), for example, describe an SCR catalyst comprising an oxide of titanium, an oxide of tungsten, and an oxide of vanadium. Vanadia/titania ammonia SCR catalysts normally operate at a temperature of approximately 250-370° C. Exhaust gas from light duty diesel vehicles is normally at a temperature of approximately 200° C. or less. Vanadia/titania SCR catalysts do not have significant activity at temperatures as low as 200° C.
Byrne (U.S. Pat. No. 4,961,917, assigned to Engelhard Corporation) disclose a method of passing ammonia, nitrogen oxides, and oxygen over iron or copper-promoted zeolite catalysts to selectively catalyze the reduction of the nitrogen oxides. The fresh copper-promoted catalyst has good activity. However, the copper catalyst deactivates significantly when aged. Although the iron catalyst is far more stable than the copper catalyst, it has maximum activity at about 350-500° C., significantly higher than the 200° C. temperatures that occur in many diesel exhaust streams.
NOx removal may be done using Lean NOx Trap (LNT) technology or using Selective Catalytic Reduction (SCR) Technology, as are known in the art. SCR is a proven technology which offers a wider temperature range for NOx removal from lean exhaust compare to LNT. PCT Published Application No. WO 2008/085265 (which describes low temperature selective catalytic reduction) is incorporated by reference. SCR is also less expensive since LNT contains significant amount of Platinum Group Metals (PGM) while SCR catalysts may be PGM-free.
Diesel particulate filters (DPFs) have been used for many years to remove the PM from diesel exhaust stream. PCT Published Application No. WO 2006/044764 (which describes diesel particulate filters) is herein incorporated by reference. The PM may comprise lube oil solids, soot and other carbon particulates, inorganic ash and mixtures thereof. PM accumulates over time within the DPF. The pressure drop through the DPF may increase as the PM levels on the DPF increase. To continue normal operation, a DPF may need to be cleaned and/or regenerated. The lube oil solids and carbon particulates can sometimes be removed from the DPF through combustion. However, the ignition temperature of the carbon particulates is normally about 600° C., and diesel exhaust temperatures are rarely that high. In order to combust the PM, the exhaust gas temperature can be increased by retarding the timing of fuel injection, but at the cost of fuel efficiency. Alternatively, the DPF can be heated with an on-vehicle electric heater, however, heating the electric heater requires energy, with an accompanying fuel efficiency penalty. Further, subjecting the DPF to increased temperature may damage the DPF by thermal cracking, or sintering of PM to the DPF. Finally, the DPF may be removed and regenerated off-vehicle, but this requires downtime for maintenance.
Placing a catalyst on the DPF to lower the oxidation temperature of the PM has recently been attempted in diesel aftertreatment systems. The benefit is that by use of a catalyst, PM may combust at normal exhaust temperatures, alleviating or reducing the need for DPF regeneration. Hartwig (U.S. Pat. No. 4,510,265, incorporated by reference) describes a catalyst comprising a platinum group metal and silver vanadate. The catalyst of Homeier (U.S. Pat. No. 4,759,918, incorporated by reference) comprises platinum, palladium, or rhodium on a sulfur resistant support such as titania or zirconia. Dettling (U.S. Pat. No. 5,100,632, incorporated by reference) utilizes a catalyst that is a mixture of one or more platinum group metals and one or more alkaline earth oxides such as magnesium oxide. The catalysts of Hartwig, Homeier, and Dettling et al. comprise platinum group metals (PGMs). PGMs includes gold, platinum, palladium, rhodium and any mixture thereof. PGM catalysts are typically expensive.
DPFs that contain vanadium catalysts to lower the combustion temperature of the carbon particulates have been described in U.S. Pat. No. 4,900,517 (incorporated by reference). Other vanadium catalysts are discussed in U.S. Pat. No. 6,013,599 (incorporated by reference). Vanadium oxides are volatile and toxic. Further, the high temperatures that are present in the DPF during combustion of the carbon particulates can vaporize the vanadium catalysts on the DPF, potentially leading to health problems in the general populace.
Exhaust gas from motor vehicles and engines such as gas turbines contains nitrogen oxides and particulate matter. Traditionally, the nitrogen oxides in the exhaust gas can be removed by contacting the exhaust gas with reducing agents such as ammonia in the presence of a selective catalytic reduction (SCR) catalyst. The ammonia or other reducing agent reacts with the nitrogen oxides to form nitrogen and water. The particulates are commonly removed with a separate filter. A typical lean burn exhaust aftertreatment system would consist of several subsystems: 1) oxidation catalyst (CO and HC removal and NO to NO2 conversion), 2) NOx removal system (e.g., SCR catalyst) and 3) particulate matter filter (e.g. diesel particulate filter).